This invention relates to a coaxial engine starter and more particularly to a coaxial engine starter for a vehicular internal combustion engine.
FIG. 1 illustrates one example of a conventional coaxial engine starter disclosed for example in Japanese Utility Model Laid-Open No. 63-71474. As shown in FIG. 1, the conventional coaxial engine starter 1 comprises a d.c. electric motor 2 having a hollow armature rotary shaft 3 having an inner passage 3a formed therein. A plunger rod of an electromagnetic switch (not shown) disposed at the rear end of the d.c. electric motor 2 is inserted into the inner passage 3a of the armature rotary shaft 3 so that the front end of the plunger is engageable with the rear end of the output rotary shaft 4 coaxially inserted into the inner passage 3a of the armature rotary shaft 3 from its front end to push forward the output rotary shaft 4.
It is seen from FIG. 1 that the output rotary shaft 4 is provided at its front end (the right end as viewed in FIG. 1) with a pinion gear 5 for engaging and driving an engine ring gear (not shown). The rear end 4a (the left end as viewed in FIG. 1) of the output rotary shaft 4 which is inserted into the inner passage 3a of the armature rotary shaft 3 is journaled by a sleeve bearing 6 fitted in the inner circumference of the inner passage 3a so that the output rotary shaft 4 is slidable in the axial direction. The rotational driving force from the armature rotary shaft 3 of the d.c. electric motor 2 is transmitted to the axially slidable output rotary shaft 4 by a drive force transmission mechanism 8 including an over-running clutch or a unidirectional clutch 7 and a planetary speed reduction gear 9.
The unidirectional clutch 7 comprises an annular clutch outer member 7a, a clutch inner member 7b disposed inside of the clutch outer member 7a and a plurality of rollers 7c disposed between the clutch outer and inner members 7a and 7b. The clutch inner member 7b has formed in an inner circumferential surface helical splines which are in engagement with a helical spline portion 4c on the outer circumference of an enlarged-diameter flange portion 4b of the output rotary shaft 4. The planetary speed reduction gear 9 comprises a sun gear 9a integrally formed around the outer circumference of the front end of the armature rotary shaft 3 and planetary gears 9b supported around the sun gear 9a by pivot pins 9c attached to the clutch outer member 7a of the unidirectional clutch 7.
In the conventional coaxial engine starter 1 of the above construction, when the output rotary shaft 4 slides forward to cause the pinion thereon to engage with an engine ring gear (not shown), the reactive force generated on the output rotary shaft 4 upon the engagement is supported by the helical spline portion 4c and the sleeve bearing 6. Therefore, the maximum bending moment on the output rotary shaft 4 is exerted on the enlarged-diameter flange portion 4b on which the helical spline portion 4c is formed, so that the stress on a step portion P or a rear end face of the flange portion 4b can become significantly large to such an extent that the step portion P is broken or damaged.